Systems and methods for varying the transmittance or reflectance of light are known in the art. Such systems are employed in automotive rear-view or side-view mirrors, automotive windows, spectacles, head-up displays (HUD), head-mounted displays (HMD), and the like.
Conventional automotive rear-view mirrors have a constant reflectance. Hence, a sudden increase in luminance, for example when the image of the headlamps of a vehicle approaching from behind is reflected toward the driver through the rear-view mirror, may temporarily blind the driver. This is especially disturbing during poor environmental lighting conditions, when the eyes of the driver have become accommodated to the reduced background luminance. In conventional viewing glasses, motorcycle or ski goggles, automotive windows, and the like, the operator is frequently exposed to excessive sun light, which may cause discomfort and even eye damage.
In HUD and HMD systems, the displayed information includes symbols, graphical or alphanumeric image components. Displaying this information on the field of view of the pilot, considerably reduces the physical tasks required during the flight, such as checking the flight instruments, aiming toward a target, landing, and the like. Similarly, in cameras or other visor assisted devices, assisting data (e.g., aiming, pointing, focusing or zooming data, exposure time, lens stop data, and the like) are superimposed on the external field of view. The operator of such devices should be able to perceive both the displayed data and the background view, without moving his eyes or head between the display and the outside scene. In conventional HUD and HMD, the background scene, which is illuminated by sunlight, is often of a very high luminance, up to several thousands of foot-Lamberts (fL), and furthermore, direct sunlight has a luminance even several orders of magnitude larger.
The term “contrast” herein below refers to the ratio of the luminance of two images at a certain viewing plane. When an image is displayed on a background scene, the contrast is equal to the ratio between the luminance of the displayed image and the sum of the luminances of the displayed image and the background scene. The higher the luminance of the background scene, the lower the contrast of the displayed image against the background scene. For example, the contrast of a displayed image having a luminance of 500 fL, which is displayed against a background scene having a luminance of 4000 fL, is about 11%. However, the same displayed image would have a contrast of about 40%, if displayed against a background scene having a luminance of 800 fL.
Hence, in such situations it is desirable to reduce the luminance of the background scene. Different techniques have been applied or proposed, for reducing the background luminance. One such technique employs a mechanism for switching between a plurality of fixed states, such as commonly applied in automotive rear-view mirrors. Other techniques employ a fixed, transparent light filter, similar to that used in sunglasses or sun visors, or a fixed opaque shield for reducing the viewing aperture, such as sun blinds. However, these devices reduce the visibility of the background scene under poor illumination.
Another technique for reducing background luminance employs auto-dimming mirrors (ADM) or variable transmittance optics (VTO) with real-time adaptive reflectance and transmittance, respectively. Mirrors or viewing glasses according to these techniques, exhibit variable reflection or transmission, respectively, whereby they can be manually or automatically switched to a dark state or a bright state, or switched between a plurality of intermediate states, which may either be continuous or discrete. The dark state, also referred to as the closed state, is the state of minimal transmittance or reflectance. The bright state, also referred to as the open state, is the state of maximal transmittance or reflectance.
Conventional ADM devices, according to one technique, include a VTO component, and a highly reflective surface. Light incident upon such a device, passes through the VTO component and is reflected by the reflective surface back to the VTO component, whereby the amount of light eventually reflected from the device depends on the state of the VTO component.
Electro-optic ADM and VTO devices may be either normally open (also referred to as normally bright), meaning they are in the open state when no electric field is applied, or normally closed (also referred to as normally dark), meaning they are in the open state when an electric field is applied. Normally open devices have the property that in case of power failure, the device continues to transmit or reflect the incident light, without substantially affecting the incident light. It is noted that this property may be critical in human eye related applications, such as automotive rear-view mirrors or windows, wherein the driver has to be able to view the images through the device, at all times. The contrast ratio, also referred to as the dynamic range, is equal to the ratio of the maximal and minimal obtainable transmittance or reflectance values of the VTO or ADM device, respectively.
VTO and ADM techniques have employed various elements, such as photochromic (PhC) materials, electrochromic (EC) cells, suspended particle technology and conventional liquid crystal (LC) technology. PhC materials have a limited spectral and photochemical sensitivity. Hence, devices using these materials are highly dependent on the incidence of strong ultraviolet illumination thereupon.
EC and SP cells have relatively large response times, typically in the order of seconds to minutes, and hence, they are not normally employed in rapidly-changing illumination conditions. Furthermore, EC materials exhibit a memory effect in their dark state, and hence, devices employing these materials require a special reverse drive circuit in order to return to the transparent default state. SP cells are normally dark by nature, and hence are applicable only to normally dark devices.
There are several conventional ADM and VTO techniques which employ the LC technology. A device, according to one such technique, includes an LC pane sandwiched between two crossed polarizers. The transmittance of the device depends on the state of the LC pane, which may be controlled by an electric field applied thereto. However, a polarizer absorbs at least 50% of unpolarized incident light, thereby determining the open state transmittance to be no more than 50%.
U.S. Pat. No. 5,015,086 issued to Okaue et al., and entitled “Electronic Sunglasses”, is directed to electronically controlled sunglasses utilizing conventional LC technology and powered by a solar cell. According to embodiment 1 of the disclosure, the LC panel used has a film substrate including an electrode surface and a nematic liquid crystal having a proper amount of right-spinning chirality material. Light-polarizing plates are pasted on both sides of the film substrate, with the absorption axes of these light-polarizing plates matching the rubbing direction of the film substrate. The maximal transmittance of the transmittance varying section is 35%.
Another technique which employs LC technology, alleviates the polarizer dependent light loss by using dichroic or pleochroic guest-host liquid crystal (GH-LC) mixtures, whereby no polarizers are needed. Devices according to these techniques, especially those with normally open cells, have generally yielded poor contrast ratios.
U.S. Pat. No. 4,660,937 issued to Richardson, and entitled “Dichroic Dye-Nematic Liquid Crystal Mirror”, is directed to an auto-dimming mirror, which utilizes a liquid crystal containing a dichroic dye. The auto-dimming mirror includes a liquid crystal material enclosed by a seal, a reflective surface, an electrically conducting layer and transparent front and back members on each side of the liquid crystal material. The auto-dimming mirror may be either normally open or normally closed, depending on whether the liquid crystal has positive or negative anisotropy and whether the dichroic dye is positive or negative. The dynamic range reported was about 3.7 for the normally closed embodiment, and about 1.3 for the normally open embodiment.
U.S. Pat. No. 6,239,778 issued to Palffy-Muhoray et al., and entitled “Variable Light Attenuating Dichroic Dye Guest-Host Device”, is directed to a VTO cell which utilizes a host material and a light absorbing dichroic dye guest. A solution of dichroic dye and a liquid crystalline material is disposed between two transmissive substrates, whose inner surfaces are coated with an electrically conducting layer. Each side of the VTO cell further includes an alignment layer and a passivation or insulating layer. The electrically conducting layers are connected to a power circuit, which includes a variable voltage supply controlling the transmittance of the cell.
European Patent Application Publication No. EP1158336A2 to Weiss et al., and entitled “System and Method for Varying the Transmittance of Light Through a Media”, discloses a system and method for varying the transmittance through selected portions of a media, on which images are displayed. The system includes a VTO media made of a non-conventional LC material, such as a dichroic dye GHLC. In one embodiment of the disclosed system, a double cell configuration is applied. Accordingly, two VTO cells are applied, which have mutually perpendicular LC director orientations, thereby effectively behaving as two crossed polarizers when no voltage is applied. When a voltage is applied, each of the cells switches to a homeotropic phase, with the director orientation perpendicular to the cell surface, thereby minimizing the dichroic dye absorption. Thus, the double-cell configuration may be used for a normally closed VTO cell.
U.S. Pat. No. 4,690,508 issued to Jacob and entitled “Liquid Crystal Closed-Loop Controlled Mirror Systems”, is directed to a rear view mirror system of an automotive vehicle, which reflects light at variable intensities. The rear view mirror system includes a mirrored reflecting surface, a liquid crystal unit, two light sensors, a pair of adjustment devices and an electronic circuit, all located within a housing. The mirrored reflecting surface is located behind the liquid crystal unit and tilted with respect to the liquid crystal unit. One of the light sensors detects ambient light and the other light sensor detects the light which reaches the mirrored reflecting surface after passing through the liquid crystal unit. The mirrored reflecting surface is tilted with respect to the liquid crystal unit, thereby preventing first surface reflections from the liquid crystal unit to from reaching the driver, where the mirrored reflecting surface was substantially parallel with the liquid crystal unit.
The electronic circuit changes the opaqueness of the liquid crystal unit in a closed control loop, according to signals received from the two light sensors, thereby changing the intensity of the light which is reflected by the mirrored reflecting surface. When the intensity of the incident light to the liquid crystal unit increases, the electronic circuit increases the opaqueness of the liquid crystal unit. Conversely, when the intensity of the incident light to the liquid crystal unit decreases, the electronic circuit decreases the opaqueness of the liquid crystal unit. When the ambient light changes from brighter to darker, the electronic circuit increases the opaqueness of the liquid crystal unit. Conversely, when the ambient light changes from darker to brighter, the electronic circuit decreases the opaqueness of the liquid crystal unit. The driver can change the opaqueness of the liquid crystal unit manually, by the pair of adjustment devices.
International Publication No. WO 02/06888 A1 published on 24 Jan. 2002 and entitled “Bistable Liquid Crystal Devices”, is directed to methods to change the direction of alignment of the molecules of a liquid crystal device. The liquid crystal device includes a layer of liquid crystal material containing a dichroic additive, sandwiched between a first substrate and a second substrate. The surface of the first substrate is treated in order to introduce surface profiles having different azimuthal directions and thus provide azimuthal bistable surface alignment. The profiles can be 90 degrees apart. The surface of the second substrate is rubbed in order to provide planar alignment.
Linearly polarized light which strikes the first surface applies a torque to the molecules of the liquid crystal material. When an electric field is applied across the liquid crystal device, the molecules produce a homeotropic orientation. When the electric field is removed, the alignment of the first substrate relaxes to the direction of polarization of the linearly polarized light. This alignment is transferred to the second substrate, such that all molecules of the liquid crystal material are aligned in the direction of polarization of the linearly polarized light, even after removal of the linearly polarized light. When a dye or a light absorptive material is incorporated with the liquid crystal material, the linearly polarized light induces local heating, which assists the alignment of the molecules with the direction of polarization of the linearly polarized light.
A report published by Dozov et al., and entitled “Fast Bistable Nematic Display From Coupled Surface Anchoring Breaking”, is directed to a bistable nematic liquid crystal, whose structure can be switched between a uniform (untwisted) stable state and a twisted stable state. This report is published in collaboration with SFIM-ODS SPIE Vol. 3011115 027-786X/97 p 61-69.
The bistable nematic liquid crystal in the uniform state, operates as a half-wave plate, so that if the bistable nematic liquid crystal is located between crossed polarizers, maximum light is transmitted. In the twisted state, the bistable nematic liquid crystal operates as an isotropic chiral optical system which introduces a relatively small rotation of the polarization (i.e., 10-30 degrees). Thus, with crossed polarizers, the bistable nematic liquid crystal in the twisted state is relatively dark.
In the uniform state, the molecules of the bistable nematic liquid crystal are uniformly parallel with the two plates of the bistable nematic liquid crystal. In the twisted state, only the layer of the molecules adjacent to the two plates are parallel to each plate and the other molecules are twisted between the two layers, thereby providing a twist of 180 degrees. The bistable nematic liquid crystal can be switched to the twisted state while passing through a homeotropic intermediate state, by applying a rapidly decreasing electric field. The bistable nematic liquid crystal can be switched to the uniform state while passing through the homeotropic intermediate state, by applying a slowly decreasing or a stepwise electric field.